I have been asked on more than one occasion for advice on nutrition during a round of golf. My reply is always to ask them "how much water do you drink?" The response is usually underwhelming, often very little and always overlooked.
Water is an essential ingredient that can improve performance. The human body needs 80oz of water a day just to perform normal bodily functions (sweat, excretion etc), yet on average we consume 32oz per day. If you consider that our brains are made of 85% water, and use it to conduct electrical energy for thought process and memory function, then surely to focus on pre-hydrating the days before you play and during the round will make a significant difference in your play. If you start the round dehydrated it will be almost impossible to address the balance during the round, especially in the summer months.
My advice is to always try and consume your 80oz/ day, although it isn't easy. But you can at least pre-hydrate for the two days leading up to a big match or tournament and drink water on every hole.